Hydrocarbons: Alkanes and Alkenes

This guide covers Topic 14, Hydrocarbons — subtopics 14.1 Alkanes and 14.2 Alkenes in full — from Cambridge International AS & A Level Chemistry 9701, 2025–2027 series. This is AS Level content.

Before studying this

At IGCSE / O Level (subtopics 11.4 and 11.5), you learned that alkanes are saturated hydrocarbons that are generally unreactive except for combustion and substitution with chlorine, and that alkenes are unsaturated hydrocarbons made by cracking, distinguished from alkanes by decolourising aqueous bromine. That level stops at naming the reaction types — it does not require a mechanism.

AS Level adds three things IGCSE does not: the step-by-step free-radical substitution mechanism (initiation, propagation, termination) for alkanes, the step-by-step electrophilic addition mechanism for alkenes (including why Markovnikov’s rule holds), and a wider set of named reactions and reagents for producing each class of compound. You should also be comfortable with the vocabulary from Organic Mechanisms: An Introduction — homolytic fission, free radical, electrophile, curly arrows — before working through the mechanisms below.

Syllabus coverage

CAMBRIDGE INTERNATIONAL AS & A LEVEL CHEMISTRY 9701 — AS Level, Topic 14

14.1 Alkanes — the reactions by which alkanes are produced (hydrogenation of an alkene with H₂ and a Pt/Ni catalyst; cracking of a longer-chain alkane with heat and Al₂O₃); complete and incomplete combustion; free-radical substitution by Cl₂ or Br₂ in the presence of ultraviolet light, exemplified by ethane; the mechanism of free-radical substitution (initiation, propagation, termination); how cracking produces more useful alkanes and alkenes of lower Mr from heavier crude oil fractions; the general unreactivity of alkanes, including towards polar reagents, in terms of C–H bond strength and low polarity; the environmental consequences of carbon monoxide, oxides of nitrogen and unburnt hydrocarbons from combustion in the internal combustion engine, and their catalytic removal.

14.2 Alkenes — the reactions by which alkenes are produced (elimination of HX from a halogenoalkane using ethanolic NaOH and heat; dehydration of an alcohol using a heated catalyst such as Al₂O₃ or a concentrated acid such as concentrated H₂SO₄; cracking of a longer-chain alkane); electrophilic addition with hydrogen (H₂/Pt or Ni, heat), steam (H₂O(g)/H₃PO₄ catalyst), a hydrogen halide (HX(g), room temperature) and a halogen (X₂); oxidation by cold dilute acidified KMnO₄ to form a diol; oxidation by hot concentrated acidified KMnO₄, rupturing the C=C bond, and using the products to locate the position of an alkene linkage in a larger molecule; addition polymerisation, exemplified by ethene and propene; the use of aqueous bromine to test for a C=C bond; the mechanism of electrophilic addition, using bromine/ethene and hydrogen bromide/propene as examples; the inductive effect of alkyl groups on the stability of primary, secondary and tertiary carbocations formed during electrophilic addition, used to explain Markovnikov addition.

Producing alkanes and alkenes

Alkanes are produced by hydrogenation of an alkene (H₂, Pt or Ni catalyst — e.g. CH₂=CH₂ + H₂ → CH₃CH₃) or by cracking a longer-chain alkane (heat, Al₂O₃ catalyst), which also produces shorter alkenes as co-products and turns heavier, less useful crude oil fractions into lighter, more useful ones. Alkenes are produced by elimination of HX from a halogenoalkane (ethanolic NaOH, heat), by dehydration of an alcohol (a heated catalyst such as Al₂O₃, or a concentrated acid such as concentrated H₂SO₄), or by cracking. These production routes are why this topic links to the halogenoalkanes and alcohols resources elsewhere on this site.

Alkanes: free-radical substitution

Alkanes are saturated — every carbon–carbon bond is a single covalent bond — which makes them generally unreactive: the C–H bonds are strong and only weakly polar, so alkanes resist attack by the polar and ionic reagents that attack other functional groups. Their two reactions worth knowing are combustion and substitution by chlorine or bromine under ultraviolet light.

Mechanism, exemplified by ethane and chlorine:

Initiation — UV light supplies enough energy to break the Cl–Cl bond homolytically:

Cl₂ → 2Cl• (each chlorine atom keeps one electron of the shared pair)

Propagation — two steps, each regenerating a radical so the chain continues:

Cl• + C₂H₆ → C₂H₅• + HCl

C₂H₅• + Cl₂ → C₂H₅Cl + Cl•

Termination — any two radicals combine, removing them from the chain:

Cl• + Cl• → Cl₂

C₂H₅• + C₂H₅• → C₄H₁₀

C₂H₅• + Cl• → C₂H₅Cl

Because chlorine keeps being regenerated in propagation, one initiation event can trigger many substitutions — this is why the reaction is described as a chain reaction. Further substitution beyond mono-substitution is possible in principle but only mono-substitution is required for AS Level.

Combustion of alkanes

Complete combustion, with a plentiful supply of oxygen, produces only carbon dioxide and water:

CₙH₂ₙ₊₂ + ((3n+1)/2) O₂ → n CO₂ + (n+1) H₂O

Incomplete combustion, with a limited supply of oxygen, produces carbon monoxide and/or carbon (soot) instead of some of the CO₂, because there is not enough oxygen to oxidise every carbon atom fully — for example:

2C₂H₆ + 5O₂ → 4CO + 6H₂O

Environmental consequences of combustion

Burning hydrocarbons in the internal combustion engine produces pollutants beyond CO₂ and H₂O: carbon monoxide (toxic, from incomplete combustion), oxides of nitrogen (NOₓ, formed when N₂ and O₂ from the air react together at the high temperature inside the engine, contributing to acid rain and photochemical smog), and unburnt hydrocarbons (from incomplete combustion, also contributing to smog). Catalytic converters remove all three pollutants together, converting them into CO₂, N₂ and H₂O over a platinum/rhodium catalyst.

Alkenes: electrophilic addition

The C=C double bond consists of a σ bond and a π bond. The π bond’s electron density lies above and below the plane of the molecule, exposed and accessible — this is what makes alkenes far more reactive than alkanes and what makes them susceptible to attack by electrophiles.

Alkenes undergo electrophilic addition with four reagents: hydrogen (H₂, Pt or Ni catalyst, heat) to give an alkane; steam (H₂O(g), H₃PO₄ catalyst) to give an alcohol; a hydrogen halide (HX(g), room temperature) to give a halogenoalkane; and a halogen (X₂) to give a dihalogenoalkane. The halogen and hydrogen halide additions are detailed below because they illustrate the mechanism and Markovnikov’s rule most clearly.

Mechanism, bromine and ethene: as the non-polar Br₂ molecule approaches the electron-rich π bond, the approaching electrons induce a temporary dipole in the bromine molecule (δ+/δ−). The π bond’s electrons then attack the δ+ bromine atom, forming a C–Br bond and breaking the Br–Br bond heterolytically, producing a bromide ion and a positively charged carbocation intermediate. The bromide ion then attacks the carbocation from the opposite face, completing the addition to give 1,2-dibromoethane.

Mechanism, hydrogen bromide and propene (Markovnikov addition): HBr is already polar (δ+ on H, δ− on Br). The π bond attacks the δ+ hydrogen, breaking the H–Br bond heterolytically and forming a carbocation on one of the two double-bond carbons. Two carbocations are possible — a secondary cation (on C2) or a primary cation (on C1) — and the reaction proceeds through whichever is more stable, because a more stable intermediate forms faster (lower activation energy).

Why the secondary cation is favoured: alkyl groups are electron-donating by the inductive effect, pushing electron density towards the positively charged carbon and stabilising it. A secondary carbocation has two alkyl groups donating into the charged carbon; a primary carbocation has only one. The secondary cation is therefore more stable, forms preferentially, and the bromide ion then attacks it to give the major product.

Worked example. Predict the major organic product of propene reacting with HBr, and explain the choice using inductive effects.

Propene is CH₃–CH=CH₂. Protonation of the double bond can place the positive charge on C2 (secondary carbocation, CH₃–CH⁺–CH₃) or on C1 (primary carbocation, CH₃–CH₂–CH₂⁺). The secondary carbocation is stabilised by a methyl group on each side of the positively charged carbon — two alkyl groups donating electron density into it — while the primary carbocation has only one alkyl group doing so. The reaction proceeds via the more stable secondary carbocation, so bromide attacks C2. The major product is 2-bromopropane, CH₃–CHBr–CH₃, with 1-bromopropane forming only as a minor product.

Oxidation with acidified KMnO₄: diols and locating a double bond

Cold, dilute acidified KMnO₄ oxidises the C=C bond to a diol (an –OH group is added to each of the two carbons that were double-bonded) without breaking the carbon chain — for example, ethene gives ethane-1,2-diol. This reaction is also usable as a test for unsaturation, since the purple KMnO₄ is decolourised.

Hot, concentrated acidified KMnO₄ breaks the C=C bond completely, and the identity of the fragments tells you where the double bond was: a terminal =CH₂ group is oxidised all the way to CO₂ and water, an internal =CH– group is oxidised to a carboxylic acid, and a fully substituted =CR₂ group is oxidised to a ketone. Working backwards from the products to the alkene’s structure is a standard structure-determination question.

Addition polymerisation

Alkenes undergo addition polymerisation: many alkene monomers join through their π bonds into a long-chain polymer, with no other product formed. For example, n CH₂=CH₂ → –(CH₂–CH₂)ₙ– (poly(ethene)), and propene polymerises the same way to give poly(propene). The repeat unit contains the same atoms as the monomer, just with the former C=C bond opened into two single bonds linking to neighbouring repeat units.

Testing for a C=C bond: aqueous bromine

Shaking an alkene with aqueous bromine (bromine water) decolourises it rapidly, in the dark, at room temperature, as Br₂ adds across the C=C bond to give a colourless dibromo compound. Alkanes do not decolourise bromine water under these conditions — they only react with Br₂ slowly, and only under ultraviolet light, via substitution — so this test distinguishes an alkene from an alkane without needing UV light or heat.

Common mistakes

• Writing “Cl₂ → Cl⁺ + Cl⁻” for initiation. Homolytic fission under UV light splits the bond evenly into two radicals, each with one electron — not into ions.
• Forgetting that propagation is two separate steps. Each propagation step must both consume a radical and produce a new one, keeping the chain going; a single combined equation loses this detail and the marks that go with it.
• Predicting the minor product as the major one. Markovnikov addition follows the more stable carbocation, not simply “the carbon with more hydrogens” — reason from inductive stabilisation, not a memorised shortcut.
• Using cold dilute KMnO₄ conditions where hot concentrated conditions are needed, or vice versa. Cold dilute acidified KMnO₄ stops at the diol; only hot concentrated acidified KMnO₄ cleaves the C=C bond.

Quick revision checklist

• Alkane production: hydrogenation of an alkene, cracking of a longer alkane
• Free-radical substitution mechanism: initiation, propagation (two steps), termination, exemplified by ethane and chlorine
• Why alkanes are generally unreactive: strong, weakly polar C–H bonds
• Environmental impact of combustion products and catalytic removal
• Alkene production: elimination from a halogenoalkane, dehydration of an alcohol, cracking
• Electrophilic addition with H₂, steam, HX and X₂; oxidation with cold dilute vs hot concentrated acidified KMnO₄; addition polymerisation
• Electrophilic addition mechanism for bromine/ethene and HBr/propene
• Markovnikov addition explained by carbocation stability and inductive effects

Related resources

• Organic Mechanisms: An Introduction — the mechanism vocabulary and curly-arrow notation this resource assumes
• Halogenoalkanes: Nucleophilic Substitution and Elimination — halogenoalkanes made from alkanes and alkenes, and their own reactions
• Alcohols: Reactions and Oxidation — alcohols made from alkenes by electrophilic addition
• Addition Polymerisation — the polymerisation reaction of alkenes, covered in full
• Arenes and Halogenoarenes: Electrophilic Substitution and Reactivity — the A Level aromatic hydrocarbon, benzene
• Cambridge AS & A Level Chemistry hub

Written against Cambridge International AS & A Level Chemistry 9701, 2025–2027 series. Always check the current syllabus for your examination year.